By means of the terrying apparatus in such looms for manufacturing a terry cloth web, several weft threads are partially beat-up one after another by the weaving reed to a predetermined point located a certain spacing away from the beat-up edge or interlacing point. Then, the several weft threads are together fully beat-up to the beat-up edge or interlacing point. During this procedure, the base warp threads are tightly tensioned and the weft threads glide along the tightly held base warp threads. The nap or pile warp threads, on the other hand, have a relatively lower tension and are held and pulled along by the weft threads as the weft threads are beat-up, so that rows of pile loops are formed by the nap or pile warp.
In order to produce a so-called three-shot terry cloth fabric, the process described above is carried out with two partial beat-up cycles respectively followed by one full beat-up cycle. Correspondingly, to produce a so-called four-shot terry cloth fabric, a full beat-up cycle respectively follows each three partial beat-up cycles. In such a process, the final nap or pile loop height is predominantly determined by the spacing of the several weft threads or the shot group from the beat-up edge or interlacing point of the fabric web at the time of the partial beat-up cycles.
In order to achieve a uniform visual appearance and therewith produce a high quality, high value terry cloth product, for example a terry cloth towel, it is necessary to achieve pile loop rows that are continuously uniform across the entire web length and from border to border of the web. The determinative factor for achieving such a desirable finished result is that all of the individual pile warp threads of the pile warp thread gang should have a practically uniform thread tension at each full beat-up cycle, whereby the uniform thread tension can also be a zero or null tension. It is important, as is generally known, that the pile loops formed by the respective last full beat-up are not pulled out of the woven web either during or after the full beat-up cycle due to the tension applied to the pile warp.
In known looms, the tension of the pile warp threads is momentarily reduced at the beginning of each full beat-up motion by venting the brake for the pile warp beam at the appropriate point in time. This solution is only slightly effective and is rarely used in practice today because of the substantial moment of inertia caused by the mass of the pile warp beam.
Looms are also known which have a special pile warp delivery arrangement. In such looms, the pile warp beam is momentarily accelerated during the full beat-up of the reed. This apparatus is combined with a take-off or let-off roller embodied in the form of a sand beam or sand roller or with a roller pair, by means of which, at the appropriate instant, the pile warp is let off or stripped from the warp beam at a faster rate and delivered to the loom shed in a practically tension-free state. In such a prior art solution, a substantial difficulty exists in trying to optimally match the velocity and amplitude of the additional feed advance of the pile warp to the specific requirements of the weaving process at hand, for example, to the intended length of the pile loops. It is necessary to change the speed and amplitude of the additional feed advance of the pile warp, for example, when switching over to a different weaving pattern, due to the differing requirements of different qualities of yarn or thread. This is necessary in order to reliably provide the prescribed length of pile warp thread in a practically tension-free condition during every full beat-up motion.
The German Patent Laying-Open Publication 2,318,326 discloses a method and a thread supply arrangement for pile warp threads in a loom for weaving shear nap terry cloth articles, wherein a measured pile warp thread supply is built up between every two respective successive full beat-up cycles. During the stock-piling or reserve process, the pile warp is held between the reservoir and the loom shed, in order to prevent the shear nap pile from being pulled out backwards. Furthermore, the lengths of thread necessary for forming the shear nap pile are freely released in a practically tension-free manner during the following full beat-up motion.
For carrying out the method, the reference suggests that a pile warp thread supply or reserve is built up during the time interval between two respective succeeding full beat-up cycles in an intermediate thread reservoir or thread store arranged downstream of the pile warp let-off arrangement. The intermediate reserving or storing of thread is carried out by means of a deflecting beam that can be driven through a rod linkage by a cam drive that is coupled to the reed drive of the loom. Furthermore, a cam controlled clamping arrangement for the pile warp is operatively connected to the intermediate reservoir or store. During each full or complete beat-up, the clamping arrangement and the intermediate reservoir are opened by means of the described cam drive, whereby the stored length of the pile warp is freely available in a tension-free state for forming the row of pile loops. Practicing such a method and especially using the apparatus described for the purpose is relatively costly and complicated and requires great effort, especially in view of the complex control arrangement required for providing the necessary thread storage for an entire pile warp thread gang.
German Patent Publication 2,817,185 discloses a pile warp thread let-off arrangement for a terry cloth loom, wherein the drive for the pile warp beam is controlled by a pendular swinging feeler or sensor element for the pile warp threads. The swinging sensor element is arranged parallel to and between two rotating guide idlers that are arranged at fixed locations following one another after the pile warp beam in the direction of the thread let-off. In its end positions, the sensor element actuates control switches for the pile warp beam drive. In one end position, the swinging sensor element lies essentially in a plane defined by the two guide idlers arranged at fixed locations. However, when the sensor element moves out of this end position, it deflects the pile warp threads out of their straight path, in order to form and provide the necessary pile warp thread reserve for the complete beat-up cycle. Even though only minimal pile warp tensioning forces arise due to the specific embodiment of the sensor element, and due to its low mass, it has been found that improvements in pile uniformity can still be achieved.
In comparison to various prior art approaches, the tension force applied to the pile warp must be at least large enough to completely prevent the formation of slack pile warp threads in the pile warp during the partial beat-up of the weaving reed. If this is not the case, then slack in the threads could lead to twisting and tangling among the individual pile warp threads. Thereupon, it would no longer be possible to assuredly prevent breaking of the pile warp threads. Furthermore, it must be noted that, in looms operating at high speeds, an uncontrolled swinging or oscillating of the pendular sensor element can have disadvantageous or negative influences on the formation of the pile loop rows, namely, because the pile warp threads could be partially pulled backwards after each complete beat-up motion.